Use general_blockwise in case of chunk-aligned selections in index

This allows the optimizer to perform fusion, which is not the case with
the existing `map_direct` implementation.

cubed/core/ops.py

@@ -1,6 +1,7 @@
 import builtins
 import math
 import numbers
+from dataclasses import dataclass
 from functools import partial
 from itertools import product
 from numbers import Integral, Number
@@ -25,9 +26,11 @@
 from cubed.primitive.rechunk import rechunk as primitive_rechunk
 from cubed.spec import spec_from_config
 from cubed.storage.backend import open_backend_array
+from cubed.types import T_RegularChunks, T_Shape
 from cubed.utils import (
     _concatenate2,
     array_memory,
+    array_size,
     get_item,
     offset_to_block_id,
     to_chunksize,
@@ -484,6 +487,11 @@ def merged_chunk_len_for_indexer(ia, c):
     if shape == x.shape:
         # no op case (except possibly newaxis applied below)
         out = x
+    elif array_size(shape) == 0:
+        # empty output case
+        from cubed.array_api.creation_functions import empty
+
+        out = empty(shape, dtype=x.dtype, spec=x.spec)
     else:
         dtype = x.dtype
         chunks = tuple(
@@ -494,21 +502,68 @@ def merged_chunk_len_for_indexer(ia, c):
 
         target_chunks = normalize_chunks(chunks, shape, dtype=dtype)
 
-        # memory allocated by reading one chunk from input array
-        # note that although the output chunk will overlap multiple input chunks, zarr will
-        # read the chunks in series, reusing the buffer
-        extra_projected_mem = x.chunkmem
+        if _is_chunk_aligned_selection(idx):
+            # use general_blockwise, which allows more opportunities for optimization than map_direct
 
-        out = map_direct(
-            _read_index_chunk,
-            x,
-            shape=shape,
-            dtype=dtype,
-            chunks=target_chunks,
-            extra_projected_mem=extra_projected_mem,
-            target_chunks=target_chunks,
-            selection=selection,
-        )
+            from cubed.array_api.creation_functions import offsets_virtual_array
+
+            # general_blockwise doesn't support block_id, so emulate it ourselves
+            numblocks = tuple(map(len, target_chunks))
+            offsets = offsets_virtual_array(numblocks, x.spec)
+
+            def key_function(out_key):
+                out_coords = out_key[1:]
+
+                # compute the selection on x required to get the relevant chunk for out_coords
+                in_sel = _target_chunk_selection(target_chunks, out_coords, selection)
+
+                # use a Zarr BasicIndexer to convert this to input coordinates
+                indexer = create_basic_indexer(
+                    in_sel, x.zarray_maybe_lazy.shape, x.zarray_maybe_lazy.chunks
+                )
+
+                offset_in_key = ((offsets.name,) + out_coords,)
+                return (
+                    tuple((x.name,) + chunk_coords for (chunk_coords, _, _) in indexer)
+                    + offset_in_key
+                )
+
+            # since selection is chunk-aligned, we know that we only read one block of x
+            num_input_blocks = (1, 1)  # x, offsets
+
+            out = general_blockwise(
+                _assemble_index_chunk,
+                key_function,
+                x,
+                offsets,
+                shapes=[shape],
+                dtypes=[x.dtype],
+                chunkss=[target_chunks],
+                num_input_blocks=num_input_blocks,
+                target_chunks=target_chunks,
+                selection=selection,
+                in_shape=x.shape,
+                in_chunksize=x.chunksize,
+            )
+        else:
+            # use map_direct, which can't be fused
+            # (note that it should be possible to re-write as general_blockwise with more work)
+
+            # memory allocated by reading one chunk from input array
+            # note that although the output chunk will overlap multiple input chunks, zarr will
+            # read the chunks in series, reusing the buffer
+            extra_projected_mem = x.chunkmem
+
+            out = map_direct(
+                _read_index_chunk,
+                x,
+                shape=shape,
+                dtype=dtype,
+                chunks=target_chunks,
+                extra_projected_mem=extra_projected_mem,
+                target_chunks=target_chunks,
+                selection=selection,
+            )
 
         # merge chunks for any dims with step > 1 so they are
         # the same size as the input (or slightly smaller due to rounding)
@@ -528,6 +583,72 @@ def merged_chunk_len_for_indexer(ia, c):
     return out
 
 
+def _is_chunk_aligned_selection(idx: ndindex.Tuple):
+    return all(
+        isinstance(ia, ndindex.Integer)
+        or (
+            isinstance(ia, ndindex.Slice)
+            and ia.start == 0
+            and (ia.step is None or ia.step == 1)
+        )
+        for ia in idx.args
+    )
+
+
+def create_basic_indexer(selection, shape, chunks):
+    if zarr.__version__[0] == "3":
+        from zarr.core.chunk_grids import RegularChunkGrid
+        from zarr.core.indexing import BasicIndexer
+
+        return BasicIndexer(selection, shape, RegularChunkGrid(chunk_shape=chunks))
+    else:
+        from zarr.indexing import BasicIndexer
+
+        return BasicIndexer(selection, ZarrArrayIndexingAdaptor(shape, chunks))
+
+
+@dataclass
+class ZarrArrayIndexingAdaptor:
+    _shape: T_Shape
+    _chunks: T_RegularChunks
+
+    @classmethod
+    def from_zarr_array(cls, zarray):
+        return cls(zarray.shape, zarray.chunks)
+
+
+def _assemble_index_chunk(
+    *arrs,
+    target_chunks=None,
+    selection=None,
+    in_shape=None,
+    in_chunksize=None,
+):
+    # last array contains the offset for the block_id
+    offset = int(arrs[-1])  # convert from 0-d array
+    numblocks = tuple(map(len, target_chunks))
+    block_id = offset_to_block_id(offset, numblocks)
+
+    arrs = arrs[:-1]  # drop offset array
+
+    # compute the selection on x required to get the relevant chunk for out_coords
+    out_coords = block_id
+    in_sel = _target_chunk_selection(target_chunks, out_coords, selection)
+
+    # use a Zarr BasicIndexer to convert this to input coordinates
+    indexer = create_basic_indexer(in_sel, in_shape, in_chunksize)
+
+    shape = indexer.shape
+    out = np.empty_like(arrs[0], shape=shape)
+
+    if array_size(shape) > 0:
+        _, lchunk_selection, lout_selection = zip(*indexer)
+        for ai, chunk_select, out_select in zip(arrs, lchunk_selection, lout_selection):
+            out[out_select] = ai[chunk_select]
+
+    return out
+
+
 def _read_index_chunk(
     x,
     *arrays,

cubed/tests/test_mem_utilization.py

@@ -84,6 +84,15 @@ def test_index(tmp_path, spec, executor):
     run_operation(tmp_path, executor, "index", b)
 
 
+@pytest.mark.slow
+def test_index_chunk_aligned(tmp_path, spec, executor):
+    a = cubed.random.random(
+        (10000, 10000), chunks=(5000, 5000), spec=spec
+    )  # 200MB chunks
+    b = a[0:5000, :]
+    run_operation(tmp_path, executor, "index_chunk_aligned", b)
+
+
 @pytest.mark.slow
 def test_index_step(tmp_path, spec, executor):
     a = cubed.random.random(

cubed/utils.py

@@ -10,14 +10,15 @@
 from functools import partial
 from itertools import islice
 from math import prod
-from operator import add
+from operator import add, mul
 from pathlib import Path
 from posixpath import join
 from typing import Dict, Tuple, Union, cast
 from urllib.parse import quote, unquote, urlsplit, urlunsplit
 
 import numpy as np
 import tlz as toolz
+from toolz import reduce
 
 from cubed.backend_array_api import namespace as nxp
 from cubed.types import T_DType, T_RectangularChunks, T_RegularChunks, T_Shape
@@ -41,6 +42,11 @@ def chunk_memory(arr) -> int:
     )
 
 
+def array_size(shape: T_Shape) -> int:
+    """Number of elements in an array."""
+    return reduce(mul, shape, 1)
+
+
 def offset_to_block_id(offset: int, numblocks: Tuple[int, ...]) -> Tuple[int, ...]:
     """Convert an index offset to a block ID (chunk coordinates)."""
     return tuple(int(i) for i in np.unravel_index(offset, numblocks))